The electrolysis cells with a membrane and an oxygen-reducing cathode have resulted, on the one hand, from the remarkable improvements obtained recently in terms of fluorinated ion-exchange membranes, which have made it possible to develop methods for electrolysing sodium chloride solutions by means of ion-exchange membranes. This technique makes it possible to produce hydrogen and sodium hydroxide in the cathode compartment, and chlorine in the anode compartment, of a brine electrolysis cell.
Furthermore, in order to reduce energy consumption, it has been proposed to use an oxygen-reducing electrode as the cathode, and to introduce a gas containing oxygen into the cathode compartment in order to prevent hydrogen evolution and to significantly reduce the electrolysis cell voltage.
In theory, it is possible to reduce the electrolysis voltage by 1.23 V by using the cathode reaction with supply of oxygen represented by (1) instead of the cathode reaction without supply of oxygen represented by (2): EQU 2H.sub.2 O+O.sub.2+ 4e.sup.-.fwdarw.4OH.sup.- (1)
E=+0.40 V (relative to a standard hydrogen electrode). EQU 4H.sub.2 O+4e.sup.-.fwdarw.2H.sub.2 +4OH.sup.- (2)
E=0.83 V (relative to a standard hydrogen electrode).
A conventional membrane electrolysis cell using the gas technology comprises a gas diffusion electrode (cathode) which is placed in the cathode compartment of the electrolysis cell and divides the said compartment into a solution compartment, on the ion-exchange membrane side, and a gas compartment on the opposite side.
An electrochemical cell of this type therefore generally consists of 3 separate compartments:
an anode compartment, PA1 a sodium hydroxide compartment, placed between a cation-exchange membrane (Nafion N966, Flemion F892) and the cathode, PA1 and a gas compartment.
The cathode is generally made of a silvered nickel grid covered on either side with platinized carbon.
One of the faces is coated with a fluorocarbon micropore layer in order to make it more hydrophobic.
Platinum represents 5% to 20% by weight of the carbon/platinum combination, and its average mass per unit surface area may range from 0.2 to 4 mg/cm.sup.2.
Conventional electrolysis cells with a membrane and a cathode evolving hydrogen, that is to say those employing reaction (2) mentioned above, are sometimes turned off to perform a variety of maintenance operations, or else following an incident. In such cases as these, the electrodes are de-energized, that is to say they are no longer supplied with electrical power.
Industrially, these outage phases can be managed in the following way: turning off the power and continuing the flow and addition of fluids (water and brine). The following procedure may also be adopted: turning off the power, emptying the sodium hydroxide and brine compartments, then filling with 20% strength sodium hydroxide solution (i.e. about 4 M) in the case of the cathode compartment, and with 220 g/l of brine in the case of the anode compartment (eliminating the active chlorine).
This operation is intended to preserve the performance of the membrane.
When conditions of this type are applied during outage phases to electrolysis cells with a membrane and an oxygen-reducing cathode, a significant increase in the cathode potential is observed when the electrolysis is resumed. This cathode alteration affects the voltage of the cell and leads to a significant increase in the energy consumption, which may be up to 100 kWh/tonne of sodium hydroxide produced.
Without tying applicant to an explanation, it is reasonable to assume that, in view of the simultaneous presence of oxygen and sodium hydroxide, the carbon of the de-energized cathode reacts with the oxygen and sodium hydroxide to form sodium carbonate, which deposits on the cathode. It reduces its porosity and electrical conductivity.
In order to overcome these drawbacks, Patent Application EP 0064874 has proposed a procedure which consists in completely replacing the gas (containing oxygen) in the gas compartment with nitrogen, and in keeping the nitrogen in the said gas compartment throughout the outage period.
Under these conditions, it is observed that after very short outages (a few hours), the cathode potential is little altered on restarting.